<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input:focus{
            background-color: pink;
        }
            </style>
</head>
<body>

    <h1>复合选择器</h1>
    <hr>
    <input type="text" placeholder="请输入内容">

</body>
</html>